23. Metal Oxides for CO2 Uptake
Many sorbent materials have been used to increase the efficiency of capturing and absorbing carbon dioxide, including zeolite, activated carbon, and silica. Zeolite contains compounds of crystalline aluminosilicates that are characterized by chemical and physical properties, such as selectivity and high thermal stability
[28][2]. Zeolite 13X (a bench-mark zeolite) showed absorption of up to 7.4 mmolg
−1 and higher selectivity than activated carbon. However, zeolite consumes higher temperatures for regeneration
[29][3]. Metal oxides were the most cost-effective and had lesser toxicity than zeolites and MOFs among the absorbents. In addition, the oxide-based sorbents can trap carbon dioxide with high selectivity when exposed to high temperatures
[30][4]. Usually, alkaline earth metal oxide sorbents,
for e
.g.xample, magnesium oxide (MgO) and calcium oxide (CaO), are utilized in the absorption CO
2 process. However, these methods offer a few disadvantages
[31][5]. The main drawbacks associated with the application of solid absorbents are the fast saturation and consuming energy for activation after several cycles
[32][6]. Magnesium oxide was an excellent candidate for CO
2 absorption performance
[33,34][7][8] because it had a suitable surface morphology for oxygen generation that improved absorption performance and low regeneration energy consumption
[35,36][9][10]. The incorporation of MgO-activated carbon nanofibers (ACNFs) for CO
2 absorption was successfully integrated by electrospun activated carbon nanofibers (ACNFs) using the simple volumetric method. The fusion of thermally stable metal oxides contributes to the absorption of CO
2 at its lowest temperature. The addition of ACNFs increases the production fusion of thermally stable, contributing to the absorption of CO
2 at its lowest temperature. The incorporation of MgO-ACNFs showed a higher CO
2 absorption capacity of 2.72 mmolg
−1 [37,38][11][12]. Another
study investigated magnesium oxide (MgO) nanoparticles (NPs) and MgO nanoparticles supporting activated carbon-based bamboo (BAC) for CO
2 adsorption. The MgO nanoparticles that supported BAC had surface areas of 297.1 m
2 g
−1.
TheIt study showswas showed that the physical adsorption of CO
2 on MgO(NPs)-BAC was improved by 112% (39.8 mg g
−1), compared to activated carbon-BAC (18.8 mg g
−1) or MgO nanoparticles (12.8 mg g
−1)
[39][13].
MgO with fibrous silica was utilized to improve the performance of CO
2 absorption via ultrasound-assisted impregnation
[40][14]. MgO-fibrous silica showed the highest absorption of 9.77 mmolg
−1, while fibrous silica without MgO showed an absorbance of 0.52 mmolg
−1.
One of the most extensively utilized is calcium oxide for its distinguishing features for CO
2 as a solid adsorbent. However, it has limited drawbacks, including excessive sintering and mechanical failure. The carbonation process and interaction between CaO and CO
2 slow down once the initial layer of calcium carbonate is created
[41][15]. Several
studies have attempted to improve the stability and reusability of CaO as an adsorbent by employing various strategies. CaO derived from nanosized CaCO
3 was studied by Florin et al.
[42][16]. For the carbonation of CaO derived from nanosized CaCO
3, the sorbent material was subjected to five carbonation cycles (24 h per cycle). They concluded that there was no morphological impediment when given enough time. After 100 CO
2 capture-and-release cycles of 20 min, they found that the residual conversion capacity was 20% higher than previously reported for bulk CaO, demonstrating the potential of nanosized CaO. The increase of the reactivity of the adsorbent was investigated by Li et al. using an ethanol/water mixture to hydrate CaO. They found that the spent catalyst’s sorption capacity increased by two times as much as initially, and the sorption capacity of CaO could be improved by changing the synthesis method
[43][17]. Belova et al.
[44][18] used the strategy of dispersing CaO on an inert support, such as ɣ-Al
2O
3, to improve the sorption capabilities of CaO and minimize sintering. A layer of carbonate restricts the diffusion of CO
2 and slows down the initial carbonation reaction, as previously stated. Increased dispersion reduces the possibility of sintering by increasing the surface area of the CaO particles. CaO dispersed on high surface area ɣ-Al
2O
3 resulted in a stable sorbent that could overcome the difficulties associated with limited long-term stability, slow uptake kinetics, and energy intensive regeneration
[45][19]. Thermal gravimetric analysis (TGA) was used to investigate CO
2 uptake kinetics and capacities, while multicycle experiments evaluated long-term stability. In comparison to bulk CaO powder, they found that dispersed CaO was a more effective low-temperature sorbent, with up to 1.7 times the capacity to bind CO
2. It was found that CaO dispersed on ɣ-Al
2O
3 had better long-term stability than bulk CaO, which was tested for 84 CO
2 capture-and-release cycles at 650 °C. While the bulk CaO’s adsorption capacity dropped below 50% after 20 cycles, the CaO/ɣ-Al
2O
3 adsorbent maintained 90% of its efficiency, with no sintering after the same number of cycles
[46][20].
Adding metal oxides to an amino acid, such as lysine, for CO
2 capturing is a promising process. Some groups had
studiedknown the catalytic effect of metal oxides on the salts of amino acids, which have many advantages that make them suitable alternative absorbers for carbon dioxide
[47][21] Some of these advantages are their high cyclic loading and good oxygen stability. It also allows the salt formation in potassium or lithium hydroxide, which makes the amino acid salt non-volatile in a stripper. The metal oxide catalytic activity depends on the defect sites present on the metal oxide surface
[48][22] This metal oxide surface is exposed to water, which generates metallic hydroxides, suggesting that the metal oxides can enhance the carbon dioxide absorption kinetics of the lysine salt absorbent liquid
, as shown in Figure 3.
Figure 3. Structure of potassium salt of lysine (Adapted with permission from Ref. [47]. Copyright 2018 Elsevier).
Nanoparticles, such as zinc and cobalt, have been used to increase the absorption rate of carbon dioxide, which helps reduce operating costs
[51,52][23][24]. The catalytic performance of Ni (NPs), as additives to optimize CO
2 absorption in an MEA under different mixing conditions (limited- and high-mixing), was examined using two microfluidic platforms that effectively control polyphase flows. NPs (Ni) can increase CO
2 absorption by 34% and 54% in limited- and high-mixing conditions, respectively. In addition, Ni (NPs) can reduce the amount of MEA needed in the system by speeding the lengthening time to achieve equilibrium with carbon dioxide absorption.
TheIt studywas found that Ni (NPs) could still perform its function for more than 140 h
[53,54][25][26].
The addition of TiO
2, ZnO, and ZrO
2 (NPs) were also tested with diethanolamine solution to investigate the effect on the absorption of carbon dioxide in a continuously stirred system using a stirrer bubble column, the influence of TiO
2, ZnO, and ZrO
2 nanoparticles, in aqueous piperazine solution, on the hydrodynamic and CO
2 absorption rate was examined experimentally. The absorption performance of TiO
2 and ZrO
2 nanoparticles improves with solid-loading, up to a maximum value, then declines, and the TiO
2, ZnO, and ZrO
2 nanoparticles had optimal values of 0.05, 0.1, and 0.05 (wt%), respectively. The inclusion of ZrO
2 nanoparticles had the minimum effect on the performance, compared to TiO
2 and ZnO nanoparticles, and the maximum absorption rates of TiO
2, ZnO, and ZrO
2 were 14.7%, 16.6%, and 3.7%, respectively
[55][27].
A nanocomposite of metal-organic–silica (MOS) was synthesized and functionalized, with propyl-ethylenediamine as a metal linker, for chemically attaching Cu
2+ and Ag
1+ to Cu and Ag nanoparticles for CO
2 uptake. The diameter of Cu and Ag nanoparticles ranged from 5–20 nm.
TheIt studywas showed that the composite gradually absorbed the carbon dioxide, with a saturation peak after 25 min, and incorporated metal nanoparticles with modified silica increased CO
2 uptake from 20 to 100%. In addition, silica decorated with Cu nanoparticles showed the maximum CO
2 uptake compared to Ag, Au, and Fe
[60][28].
TheIt study,was also notice
d that the effect of nanoparticles is superior to that of metal oxide nanoparticles, especially in the case of copper.
To effectively trap carbon dioxide, the surface of silica particles was chemically modified with the amine. The particles were implanted on the membranes of polyvinylidene fluoride–hexafluoropropylene (PVDF–HFP). The results demonstrated that adding amino–silica particles improved the properties of the membrane and increased the CO
2 uptake (0.8 mmol g
−1) compared to the pane membrane
[63][29].
The method of using a catalyst to capture and convert carbon dioxide is an important topic.
In this cWo
ntext, work has been done utilizing a ZnO catalyst, integrated with Ru, with amine as a novel catalyst for CO
2 capture and conversion to methanol. The ZnO catalyst assisted the reaction in a moderate condition, due to forming a ZnO–amide bond on its surface. As a result, a 30% yield has been achieved
[64][30].